This invention generally relates to radio frequency generators which accept a power such as a dc potential and convert that power into a radio frequency, higher power output. More particularly it relates to such generators which produce power which are considered especially high when compared to those typically created from a particular type of component or particular design. The invention also may involve enhanced systems and processes (including plasma processing systems, and the like) which may include a variety of ancillary features such as these higher power generators, system specific elements, or unique processes.
Definitions
Approximately Equal
In this context, voltages, currents, impedances, or other quantities are approximately equal if the differences between them are not significant in the context of the particular application or the particular design or componentry involved and, as such, do not cause an effect which is so different in that context as to make the desired end result ineffective or unsuited to the intended purpose.
Biasing
Supplying a signal, to a circuit to establish an operating point for the low frequency impedance. This signal might comprise a variety of properties, including at least a dc voltage, or a dc current, or an alternating voltage or current at any frequency.
Bias Voltage
The name given a biasing signal if it constitutes a voltage.
Bus Power
The power provided to a power amplifier circuit which is to be converted by that circuit to radio frequency power.
Bus Voltage
The voltage provided by the bus power circuit.
Combiner
An element which achieves combining.
Combining
Utilizing together the signals of a plurality of devices, such as power amplifiers to produce one or more higher power signals. The combining may be done by transformer action, by the simple summing of currents into one or more common nodes, or any other appropriate mechanism. It would include (but not be limited to) such utilization as adding, combining, summing, or totaling.
Combining Circuitry
Any element or collection of elements which cause combining, such as from power amplifiers or from other combining circuitry.
Common Electrical Point
An electrical location, such as a node or a collection of locations in an electrical circuit which receives currents or power from other locations or nodes. The common electrical point may, through the action of one or more combiners or other circuit elements act to achieve combining.
Dividing a Potential
Causing, by any mechanism, a potential to be shared among a plurality of elements. The sharing may be even, such that each element has impressed across it substantially the same potential, or it may be uneven; in a power supply context, the concept includes that the circuit is so arranged that each load has impressed across it less than the total power supply potential.
Electromagnetic Coupling Element
An element of a plasma system which is used to couple radio frequency power to a plasma.
Establishing a Low Frequency Impedance
Creating a low frequency impedance for a circuit or collection of components through any available mechanism, including but not limited to the selection of components, varying of component values, or biasing of active elements.
Radio Frequency
An electrical alternation rate high enough that the inductance and capacitance of ordinary connection elements becomes significant. This may include at least any frequency between the broad limits of 300 kHz to 500 MHZ, and may even focus on the ISM frequencies allocated by the FCC for use by equipment designed for the Industrial, Scientific, and Medical fields, or even more specifically the frequencies of 6.78 MHz, 13.56 MHz, 27.12 MHz, or 40.68 MHz.
Radio Frequency Drive Signal
A signal which alternates at a radio frequency and which is applied to the control element of a transistor or switch element to cause the transistor to vary in conductance, or to cause the switch element to transition from an xe2x80x9cONxe2x80x9d state to an xe2x80x9cOFFxe2x80x9d state.
High Power
A level of power too large to reasonably obtain from a low voltage single phase power line; in one context any power over about 100 watts would be considered xe2x80x9chighxe2x80x9d, and may even focus on the several-kilowatt range, or even more specifically 1 to 10 kW.
Impressing a Potential
In this context, arranging a circuit so that a potential appears across the object. That is, the object is subjected to the impressed potential. This might be done in a variety of ways, including at least by direct connection to the potential or by an indirect method such as passing a current through the object to create the potential.
In-Phase Components
Two signals are said to be in phase if the difference in the instants of their starting and ending alternations is small compared to their period of alternation. Two signals which are not in phase can be mathematically decomposed into fractions which are in phase and fractions which have alternations shifted by one-quarter of a period; the former are called the xe2x80x9cin-phase componentsxe2x80x9d.
Low Frequency Impedance
The ratio of voltage across an object to the current through it, measured at a frequency below the radio frequency definition.
Phase
a) The fraction of a complete cycle of an alternating signal elapsed as measured from a specified reference point; often expressed as an angle, or b) one of a multiple of power leads in a multi-phase supply system.
Series String
A connection between a plurality of elements made such that substantially most of the current from each must pass through each of the others.
Single Phase Power
A system of power distribution utilizing two wires, such as a feed wire and a return wire, between which exists an alternating voltage. Single phase power systems are often used in domestic and office environments wherein low powers are required, and may be contrasted with multi-phase power systems, which have at least three wires and are often used in high power industrial environments.
Switchmode
A circuit in which the main power element is predominately in either an xe2x80x9cONxe2x80x9d state, wherein the voltage across the element is small compared to the xe2x80x9cOFFxe2x80x9d state voltage; or in an xe2x80x9cOFFxe2x80x9d state, wherein the current passing through the element is small compared to the xe2x80x9cONxe2x80x9d state current; and wherein a condition of simultaneous substantial voltage and current is transient.
Tiered Combiner
An element which effects tiered combining.
Tiered Combining
Utilizing together the outputs of a first grouping of devices, such as power amplifiers into a first group (which may be termed level one), followed by combining the outputs of the first group into a second group (level two), continuing (if necessary and desired) until one or more final outputs are formed. The groupings may be in threes, fours or any other grouping, but are commonly in pairs. For example, eight amplifier outputs may be combined in pairs at level one into four nodes, the four nodes combined in pairs at level two into two nodes, and these last two combined in pairs at a third level into a single output. In this example, the level two outputs could be left separate as dual independent outputs without violating the concept of tiered combining.
Originally, except for some early use in diathermy machines, high power radio frequency generators were used principally as sources of energy for radio frequency transmitters. Today, operating at frequencies from about 100 kHz to about 500 MHZ, radio frequency generators are widely used for plasma processing, as well as the energy source for lasers, high frequency lighting, and dielectric heating or sealing equipment, and for providing the accelerating potential in electron and ion accelerators.
Virtually all radio frequency generators for these purposes employ an element which converts direct current (dc) power, or steady voltage, to radio frequency power. This element is called an xe2x80x9cinverterxe2x80x9d or xe2x80x9cconverterxe2x80x9d at lower frequencies, but at higher frequencies it is often termed a Power Amplifier, or PA, for historical reasons. Note that as used here the term xe2x80x9camplifierxe2x80x9d includes, but is not limited to, circuits, such as switchmode xe2x80x9camplifiersxe2x80x9d, which may not literally xe2x80x9camplifyxe2x80x9d in the sense of producing a higher power replica of an input signal. Some such circuits may simply have power leads, across which is impressed the dc potential conveying the dc input power; input terminals, across which is placed the input, or drive signal; and output terminals, across which appear the output power. The power leads could also carry non-dc power in its strictest sense so long as power from such leads is converted by the amplifier.
In most of these uses, control is required of the amount of radio frequency power produced by the generator. To provide this control, a measurement element is used to measure the output of the generator, the output of this element is compared to a desired value, and a control element which controls the output power is signaled to increase or decrease the output power to achieve the desired value.
Designers of radio frequency generation equipment have taken two approaches in the design of the control element used to control the power; usually only one is used in any given piece of equipment. The first approach controls the input, or xe2x80x9cdrivexe2x80x9d, signal to the Power Amplifier (PA) in the generator. In this case the dc power to the PA (sometimes called the xe2x80x9cbusxe2x80x9d power) is held constant. In the second approach the drive signal to the PA is held constant and the voltage of the bus power (the xe2x80x9cbus voltagexe2x80x9d) is varied as required to produce the desired output power. Such approaches for radio frequency designs may be considered distinct from the approaches often available in low frequency designs because the low frequency designs often have the advantage of not having to usually include the lead inductance and the like of ordinary connection elements. In fact, this aspect has resulted in designers recognizing a distinction, namely, that the techniques of low frequency design often do not work in the radio frequency field. As but one example, it may even be considered that concepts such as floating an amplifier or the like may be often avoided in the radio frequency field since parasitic elements can significantly alter the actual effect achieved.
In the radio frequency field, generally the former approach (varying the drive signal) is less expensive to implement, because providing a variable voltage dc power supply for the PA bus is more costly than providing for a variable drive signal. Most transistors suitable for radio frequency cannot withstand very high voltages, however, and this limits the maximum bus voltage. In generators to be used at low power levelsxe2x80x94a few tens of wattsxe2x80x94and in the United States or Japan, it is possible to rectify the 120 or 100 volt mains power directly to provide the bus voltage, but at higher power levels, or for European use, one must design for higher mains voltages. Direct rectification of US industrial and European mains voltages produces nominal dc bus voltages of 300 and 570 volts respectively, and under common conditions these values can be exceeded by 10-15%. It should also be noted that the radio frequency signals across the transistors exceed the actual dc supply voltage by an amount which depends upon the circuit configuration; in many common circuits the peak RF voltage across the transistor can be more than three times the dc supply potential. Finally, transients occur in some applications which further add to the total voltage appearing across the transistor. All of these effects add to produce peak voltage levels which can be too large for typical radio frequency transistors to withstand.
In addition to these problems related to the transistors, in the commonly used xe2x80x9cpush-pullxe2x80x9d circuits a transformer is utilized which typically contains ferrite cores, and the losses in these cores can become unmanageable at high dc supply potentials.
As a result, transformers are usually placed in the power line to reduce the voltage prior to rectification, permitting lower, safe peak voltages on the transistors. These transformers are heavy and bulky in high power circuits which we here define as those over about 100 watts. It is principally the bulk and weight of the transformer that gives rise to the second approach, use of a switchmode dc power converter. A switchmode dc power converter can be made which may operate from the directly rectified power mains and which may produce an appropriately reduced voltage, and such a power converter can be made small and light in weight. Such power converters are usually costly due to their complexity and this constitutes a disadvantage. Since a switchmode dc power converter intrinsically contains the circuitry to vary its output, however, once the designer has made the choice to incorporate such a costly element, the second power control approach becomes feasible at little or no additional cost. This second power control approach does solve another problem found in the fixed voltage method first described, which is that with a fixed power supply the power dissipation in the transistor devices can become excessive into some mismatched loads. For this reason, control circuitry can be commonly added to fixed voltage designs to limit the open circuit voltage or short circuit current in order protect the transistor devices from damage. In the second power control approach the power dissipation in the transistor devices is reduced into mismatched loads because the power supply voltage can be lowered to protect the transistor devices. The effect of this is an ability on the part of a generator designed with a variable power source to deliver more open circuit voltage or short circuit current than would be possible with a fixed voltage power source, given the same number of transistors with the same power dissipation ratings in each. This additional ability can be important in plasma powering applications and the like, because ignition of the plasma can often require significant open circuit voltage from the radio frequency generator.
So heretofore the designer of such high power radio frequency generation equipment has in many cases been faced with the following dilemma: accept the large and heavy result of incorporation of a line transformer or incorporate a switchmode power converter with its high cost. This dilemma can exist regardless of the need to control the output of the equipment.
The present invention may be considered to skirt this dilemma. It demonstrates one way to create a circuit configuration which causes the high voltage of the directly rectified power mains to be shared among two or more Power Amplifiers. This can permit the complete elimination of the bulky and heavy power transformer and the alternative of the costly voltage-reducing switchmode power converter. This may be considered a fundamentally simple concept and can be used with low frequency power supplies. But what is surprising is that this concept can be applied at radio frequency with simple circuit configurations. It is a measure of their nonobviousness that the present invention has lain undiscovered in the face of the above dilemma, in spite of years of development of the radio frequency power generator. In fact, many currently available commercial products could be significantly cost-reduced by following the teachings of the present invention.
To make this point clearer, it has been common for a high power radio frequency generator to be composed of a number of Power Amplifiers, each producing a fraction of the total output power. This has been done because, until recently, semiconducting transistors capable of producing powers much above about 300 watts were not practically available, and those that exist may require special circuits to make use of them. Thus such generators producing over a few hundred watts have used several Power Amplifiers with their outputs added, or combined, to produce the total power required. In all commercial generators in production today, these amplifiers are usually powered by a single low voltage dc supply, produced by a transformer or switchmode dc power converter. In several of these units, a simple reconnection of the power leads of the amplifiers in accordance with the invention herein disclosed would have permitted complete elimination of the transformer, which in turn would have significantly lowered the size and weight of the unit and, importantly, the cost of manufacture. That this has not been done for these designs, many of which have been on the market in their present form for over a decade, is an indication that those skilled in the art have heretofore failed to appreciate the possibilities for solution of this problem, and is a further indication of the nonobviousness of the present invention.
It is an object of this invention to provide a cost effective, reliable, simple design for a plasma power generator.
It is a further object of the present invention to provide a design which affords manufacturing and commercial advantages. In keeping with this object it is a goal to provide a design which eliminates the need for power conversion components in the dc power supply to the radio frequency Power Amplifier or inverter.
It is also a general object of the present invention to provide a design for a radio frequency power generator which is simple and cost-effective. In keeping with this object it is a goal to reduce the complexity and number of components in the power generation circuitry, and to provide a design which can utilize low cost high frequency transistors while providing a high power radio frequency output.
It is a further object of the present invention to provide a design for a high power radio frequency generator which can be smaller and lighter in weight than prior art generators. Naturally, this may enhance the scope of application of such generators.
It is an additional object of the present invention to provide a design for a high power radio frequency generator which is more reliable than prior art generators. The present invention accomplishes this object by reducing the number and complexity of the power components, and also by reducing the voltage stress to which the key semiconductor elements are subjected.
It is yet a further objective of the present invention to provide a design for a high power radio frequency generator which can achieve stable yet faster control over its output power than was possible using prior art techniques.
The present invention also has the object of providing more stable performance into load impedances which are not at the design center for the amplifier, with less dissipation or energy loss than prior art designs. In addition, techniques are disclosed for providing more power into these sub-optimal load impedances. As heretofore mentioned, this can be important to achieve ignition in plasma powering applications.
It is yet another object of the present invention to provide a design for a variable frequency high power radio frequency generator which is capable of delivering high power into a single adjustable tuned circuit, which permits easier and more accurate tuning than in prior art designs which use multiple tuned circuits.
Accordingly, the present invention provides novel designs which utilize directly rectified mains power in powering relatively lower voltage Power Amplifiers, and may thus eliminate both the bulky and heavy line transformer and the alternative costly switchmode power converter. In one embodiment, it may do this by arranging the circuits so that the rectified mains voltage (the xe2x80x9cbusxe2x80x9d power) is divided among a number of PA units. The present invention also discloses a method of providing drive to the PA units which is not only simple but provides more output power for each PA unit than prior art drive techniques. The present invention also discloses methods of providing power connections to the PA units with the property of stabilizing the PA against spurious oscillations. The present invention also discloses simple methods of combining, summing, or adding together, output signals from a plurality of amplifiers, and discloses how such a plurality of amplifiers can be arranged to drive a single output tuned circuit so that tuning of the combined array is made simple and easy. Finally, a method is presented to permit higher open circuit voltage and short circuit current from a radio frequency generator operated from a fixed-voltage power source.